The present invention relates to improving the signal to acoustic background noise ratio for voice or other audio signals picked up by acoustic transducers.
Noise-canceling microphones are a known type of prior art transducer used to improve signal to background noise ratio. The prior art noise canceling microphone operates by pressure difference, wherein the wanted source, for example the mouth of a human speaker, is much closer to the microphone than more distant noise sources, and therefore the acoustic pressure difference from the front to the back of the microphone is small for the distant sources but large for the nearby source. Therefore a microphone which operates on the pressure difference between front and back can discriminate in favor of nearby sources. Two microphones, one at the front and one at the back may be used, with their outputs being subtracted.
One disadvantage of the prior art noise canceling microphone is that it requires very close proximity (e.g. 1″) to the wanted source. Another disadvantage is that the distance from front to back of the microphone, which may be 1″ for example, causes phase shifts at higher frequencies that result in loss of discrimination at frequencies above 1 KHz
As an improvement over the noise canceling microphone, the prior art contains examples of using arrays of microphones, the outputs of which are digitized to feed separately into a digital signal processor which can combine the signals using more complex algorithms. For example, U.S. Pat. No. 6,738,481 to present inventor Krasny et al and filed Jan. 10, 2001 describes such a system, which in one implementation divides the audio frequency range into many narrow sub-bands and performs optimum noise reduction for each sub-band.
The dilemma with arrays of microphones in the prior art however is that either of the following is usually true: (a)
To avoid the clutter of multiple microphone cables, the microphones are located close together. However, if the microphones have a spacing less than half an acoustic wavelength (6″ at 1 KHz) the effectiveness of the array processing is reduced. Even just two microphones spaced 6″ apart however implies a large device; larger, for example, than a modern mobile phone (b) If widely spaced microphones are used, then the clutter and unreliability of extra cables becomes a nuisance.
Thus there is need for methods and devices that overcome the main disadvantages of the need either for extra microphones or a for multitude of extra cables in the prior art outlined above.